Allies should seek United Nations approval to arm and train Libyan rebels, a former head of the UK armed forces said, amid reports that the regime was using cluster bombs in its assault on Misrata.
Lord Dannatt said that while the Nato-led air strikes had enjoyed some success, equipping the opposition to fight effectively was vital to prevent a vacuum forming that could be filled by extremists.
And the ex-chief of the defence staff said Muammar Gaddafi's illegitimacy as leader had been firmly underlined by the apparent use of controversial cluster munitions against residential areas.
An international pressure group presented photographic evidence that the bombs - banned by more than 100 countries - were being used in Gaddafi's powerful assault on Misrata.
New York-based Human Rights Watch (HRW) said it witnessed at least three cluster bombs explode on Friday night, inspected the remains of one and spoke to ambulance drivers who attended the scene of an explosion.
It said they were Spanish-produced MAT-120 120mm mortar projectiles, which open in mid-air and release 21 submunitions over a wide area, posing a "huge risk" to civilians.
The regime denied using the controversial weapons.
The pummelling of Misrata by forces loyal to Gaddafi has become a focal point for rebel complaints that international allies are not doing enough to meet their UN mandate to protect civilians.
Nato secretary general Anders Fogh Rasmussen has said he expects member nations to provide extra ground-attack aircraft to strike regime forces, despite a meeting of the alliance ending with no concrete commitments.
Tensions over the operation are high in the UK, with Prime Minister David Cameron facing calls for Parliament to be recalled over claims he and fellow leaders had overstepped the limits of the mission by advocating regime change.